Use of a phase shift method described in non-patent literature 1 or 2 has been known as a technology of observing an image of an object to obtain a three-dimensional image of the object. In an observation device according to the phase shift method, light with a wavelength of which is emitted from a light source divided in two light components, one of which permeates the object and becomes object light and the other one of which becomes reference light, so as to take a two-dimensional image made by an interference between the object light and the reference light. An optical path length of the reference light is eccentric by λ/4 so that four two-dimensional images are taken, a specified arithmetic is performed for the four two-dimensional images so as to obtain two-dimensional complex amplitude, and then a three-dimensional amplitude image and a three-dimensional phase image of the object are obtained from a plurality of complex amplitude images to be obtained from each direction of light incident to the object.
Further, a Hilbert transform method described in a non-patent literature 3 has been known as a technology of obtaining a complex amplitude image from one captured image.